An organic EL device is a self-luminescence device utilizing the principle that a fluorescent substance emits light with recombination energy of holes injected from an anode and electrons injected from a cathode. Subsequent to the reports of a low voltage driven organic EL device with a stacked device by C. W. Tang, et al., Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, vol. 51, p. 913 (1987), etc.), studies on organic EL devices using an organic material as a constitutional material have been made actively.
For example, Patent document 1 to Patent document 4 disclose a diamine compound having a fluorene skeleton between two nitrogen atoms, and disclose an organic EL device that uses the diamine compound as a material for a hole transporting layer “adjacent to a light emitting layer”, thereby suppressing crystallization of the hole transporting material due to heat generation on light emission of the light emitting layer or the like, the organic EL device being improved in stability and durability as compared to a diamine compound having a biphenylene group between two nitrogen atoms and a monoamine compound having a fluorene skeleton.
Patent document 5 discloses production of an organic EL device having a low driving voltage and a long service life, by using a diamine compound having two nitrogen atoms bonded through a biphenylene group as a material for a first hole transporting layer and an aromatic amine derivative having a dibenzofuran structure and a carbazole structure as a material for a second hole transporting layer adjacent to a light emitting layer. Patent document 6 discloses a phosphorescent organic EL device that uses a diamine compound having two nitrogen atoms bonded through a biphenylene group in a first hole transporting layer and an amine compound having a particular heteroaryl structure in a second hole transporting layer, whereby the second hole transporting layer has electron blocking property, electroresistance, and hole injection and transporting property, thereby achieving a high efficiency and a long service life for the organic EL device. Patent document 7 discloses the use of a compound having a carbazole ring structure in a hole transporting layer “adjacent to a light emitting layer”, thereby providing an organic EL device having a high light emission efficiency and a low driving voltage.
In summary, an organic EL device, particularly a phosphorescent device, has been improved in device capability by using a hole transporting layer having a two-layer structure including a first hole transporting layer and a second hole transporting layer, and using a material having a higher capability in the second hole transporting layer “adjacent to a light emitting layer”.
The capability demanded for the second hole transporting layer includes that (i) the layer has a large triplet energy (preferably 2.6 eV or more) for preventing the excitation energy of the phosphorescent layer from being diffused, (ii) the layer has electroresistance since the layer is adjacent to the light emitting layer, (iii) the layer is an organic layer that has a small affinity (preferably 2.4 eV or less) for preventing electrons from being leaked from the light emitting layer, and (iv) the layer is an organic layer that has a large ionization potential (preferably 5.5 eV or more) for facilitating hole injection to the light emitting layer. As a material that satisfies these characteristics, a molecular skeleton having high electroresistance having a triphenylamine skeleton bonded to a heteroaryl ring, such as carbazole and dibenzofuran, is preferably used.
The first hole transporting layer is generally demanded to have excellent hole injection property to the second hole transporting layer.
For enhancing the hole injection property, it has been studied to add a compound having a p-type semiconductor property (which may be referred herein to as an acceptor material) as a hole injection layer (see Patent document 8 and Patent document 9).